Drug interactions
Diphenhydramine + Isocarboxazid
Severity: Major
Diphenhydramine and Isocarboxazid: major interaction.
Last updated Sources: UK SmPC
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
Combining a monoamine oxidase inhibitor with diphenhydramine can intensify and prolong anticholinergic effects such as dry mouth, urinary retention, blurred vision, tachycardia and confusion. Additive sedation and impaired psychomotor performance are also likely, which is of particular concern in older patients.
What this interaction means in plain words
Diphenhydramine and Isocarboxazid can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for.
Diphenhydramine belongs to Respiratory; Isocarboxazid belongs to Central Nervous System.
Why it happens
Additive anticholinergic and CNS depressant effects, with MAO inhibition potentially prolonging antihistamine anticholinergic activity.
What to do
Avoid routine use of sedating antihistamines with isocarboxazid and choose a non-sedating, low-anticholinergic alternative. If unavoidable, use the lowest dose and monitor for excessive sedation and anticholinergic toxicity.
What to ask your pharmacist
- Does the benefit of taking both outweigh the risk in my case?
- Do I need any blood test, ECG or other check while on both?
- What early warning signs should I watch for, and what should I do if they appear?
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References
Other interactions of Isocarboxazid
- 5-HTP + Isocarboxazid Contraindicated
- Amitriptyline + Isocarboxazid Contraindicated
- Amobarbital + Isocarboxazid Contraindicated
- Amoxapine + Isocarboxazid Contraindicated
- Apraclonidine + Isocarboxazid Contraindicated
- Atomoxetine + Isocarboxazid Contraindicated
Other interactions of Diphenhydramine
Frequently asked questions
Can I take Diphenhydramine with Isocarboxazid?
MediHelp rates the interaction as major. Diphenhydramine and Isocarboxazid can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for. Combining a monoamine oxidase inhibitor with diphenhydramine can intensify and prolong anticholinergic effects such as dry mouth, urinary retention, blurred vision, tachycardia and confusion. Additive sedation and impaired psychomotor performance are also likely, which is of particular concern in older patients.
How serious is the interaction between Diphenhydramine and Isocarboxazid?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Avoid routine use of sedating antihistamines with isocarboxazid and choose a non-sedating, low-anticholinergic alternative. If unavoidable, use the lowest dose and monitor for excessive sedation and anticholinergic toxicity.
What other medicines interact with Isocarboxazid?
MediHelp also lists interactions of Isocarboxazid with, for example, 5-HTP, Amitriptyline, Amobarbital. The full list is on the Isocarboxazid page.
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